1. Field of the Invention
The present invention relates to a method of repairing or modifying turbine blades of the type in which an array of turbine blades, arranged in a rotor for example, have their outer ends interconnected by cover bands, shrouds, pins or other interconnecting members.
Each blade in the array will be provided with some formation, for example an aperture or projection which cooperates with a part of the cover band, pin or shroud so as to locate the cover blade or other interconnecting member relative to the end of the blade or blades to which it is affixed.
In use of the turbine, some movement can occur between the cover bands and the ends of the blades which, in view of the high forces to which the blades are subjected, can lead to work hardening of the area of the blade in proximity to the cover band which in turn can lead to cracks and the effect known as stress corrosion cracking.
Such cracks are known in particular to occur around holes in the turbine blade tips into which projections from the cover band or other interconnecting members are secured, such cracks extending radially outwardly from the through bore or slot provided in the turbine blade.
Considerable problems have been experienced in trying to repair such cracks in view of the hardening that has taken place to the material. In some cases such is the degree of hardening that it is not possible to return the material, by heat treatment process for example, to an acceptable state.
In some cases such turbine blades may be provided with an erosion shield which requires repair, or may merely require repair to the leading edge which may have suffered erosion and the application of considerable heat as is necessary in a welding operation may itself cause damage to the end of the blade in view of the hardening that has taken place.
In practice therefore, a satisfactory repair was not possible and the whole blade had to be replaced.
It has been proposed in British patent specification No. 2091140B to solve the problem of preforming a turbine blade tenon with a root part, the root part is subsequently attached to the blade by a welding or brazing operation. The root part is preferably of elongate form and is of a shape so that it may key-in in a preformed slot in the blade itself so as to enable proper location of a tenon on the blade.
Whereas such a method of repair does overcome many of the problems of building up worn tenons by welding, the machining of the slot to receive the new tenon has to be very accurate which may considerably increase the time necessary to repair a turbine blade and involve the services of specialized persons and also utilize highly accurate machining equipment.
2. Prior Art
It has been proposed in British patent application 8818617.6 to preform an end blade portion including new tenons and to remove a piece of the original blade carrying the worn tenon and by very accurate locating means, hold the preformed part exactly in the correct position to enable securement of the new tenon and blade part to the original blade.